1. Field of the Invention
Embodiments of the invention relate to semiconductor processing in general, and in particular to a system for manipulating a workpiece during semiconductor processing applications.
2. Discussion of Related Art
Ion implantation is a process of depositing chemical species into a substrate by bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is important for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energy levels.
FIG. 1 depicts an ion implanter system. The ion implanter 100 includes a power source 101, an ion source 102, extraction electrodes 104, a 90 magnet analyzer 106, a first deceleration (D1) stage 108, a 70 magnet analyzer 110, and a second deceleration (D2) stage 112. The D1 and D2 deceleration stages (often referred to as “deceleration lenses”) are each comprised of multiple electrodes with a defined aperture to allow an ion beam to pass through. By applying different combinations of voltage potentials to the multiple electrodes, the D1 and D2 deceleration lenses may manipulate ion energies and cause the ion beam 10 to hit a target workpiece 114 at a desired energy. A number of measurement devices 116 (e.g., a dose control Faraday cup, a traveling Faraday cup, or a setup Faraday cup) may be used to monitor and control the ion beam conditions. Although not shown in FIG. 1, the target workpiece 114 may be supported by a platen which can be used to fix and to move the workpiece during implantation.
To obtain a uniform distribution of dopant ions, an ion beam is typically scanned across the surface of a target workpiece. For example, an ion beam may be scanned back and forth in the X direction while the workpiece is translated in the Y direction, thereby forming a zigzag beam path pattern.
As the semiconductor industry is producing devices with smaller and smaller feature sizes, ion beams with lower energies are required for wafer implantation. Compared with high- or medium-energy ion beams, low-energy ion beams present some unique challenges. For example, a low-energy ion beam usually has a large beam spot, which can cause problems for beam utilization and uniformity tuning. In addition, the shape or current density of a low-energy ion beam can change substantially as it propagates down a beam line. As a result, if different portions of a wafer meet the low-energy ion beam at different positions along the beam line, different dopant profiles may be created for the different portions of the wafer.
This problem is illustrated in FIG. 2, wherein an ion beam 202 is propagating along the Z direction. If a workpiece (here, wafer 204) meets the ion beam 202 at a first position Z=Z1, the wafer 204 will see a beam spot 206. If the wafer 204 meets the ion beam 202 at a second position Z=Z2, for example, when the ion beam 202 is scanned across the surface of the wafer 204, the wafer 204 will see a beam spot 208 that may be quite different from the beam spot 206. Therefore, the portion of the wafer 204 implanted with the ion beam at beam spot 208 may have a different dopant profile from the portion implanted with the ion beam at beam spot 206. The problem of beam-line variation is not unique to low-energy ion beams but can also be observed in some high-current ion beams.
As a countermeasure to the beam-line variation problem, a concept known as “isocentric scanning” has been used. Isocentric scanning generally involves moving a wafer with respect to an ion beam in such a way that the point in space where the ion beam strikes the wafer surface remains the same no matter which part of the wafer surface meets the ion beam. FIG. 3 illustrates a complex movement of a tilted wafer 304 for isocentric scanning by an ion beam 302. The ion beam 302 propagates along the Z direction. For isocentric scanning, the wafer 304 is moved in such a way that the same beam spot (or ion beam cross section) 306 strikes the surface of the wafer 304. Since the wafer 304 is tilted with respect to the ion beam 302, the wafer 304 is moved not only in the X-Y plane, but also in the Z direction. For example, at position A, the center of the tilted wafer 304 may be at Z=ZA. At position B, the center of the tilted wafer 304 may be at Z=ZB (ZB # ZA). The movement in the Z direction has to precisely coordinated with the movement in the X-Y plane in order to meet the isocentric scanning requirement.
To precisely control the tilting and the three-dimensional (3-D) translation of a wafer for isocentric scanning, existing ion implantation systems have to be equipped with a fairly complex end station to hold, tilt, and move the wafer. Such mechanisms must be capable of moving a wafer in all three dimensions. Such capabilities necessitate a sophisticated design of an end station that is often expensive to build and maintain. For example, current wafer scan systems use bearings having one or two degrees of freedom to achieve a desired wafer movement during implantation. To achieve a third degree of freedom, an additional bearing is added to the assembly, or an additional linear or rotary system is employed. Such arrangements add complexity and cost to the system. It would be desirable to provide an improved arrangement for performing isocentric ion beam scanning which overcomes the described inadequacies and shortcomings.